(1) Field of the Invention
The present invention is directed to a fluorescent material having a characteristic of producing an afterglow of high luminosity for a substantially long period of time, and a lamp having a characteristic of producing an afterglow of high luminosity for a substantially long period of time. In particular, the present invention is directed to a fluorescent material having a characteristic of producing an afterglow of high luminosity across a broad chromatic range, and a lamp having a characteristic of producing an afterglow of high luminosity across a broad chromatic range.
Furthermore, the present invention relates to a light-emitting device that can be used as a backlight source, light-emitting diode (LED) display, signal lamp, indicator, or light source in various applications, and to a display unit equipped with it. In particular, the present invention relates to a light-emitting device containing a fluorescent material that emits light by at least partially converting the light of an LED chip and having a characteristic of highly efficiently producing an afterglow of high luminosity irrespective of the environment, and to a display unit equipped with it.
(2) Description of Related Art
Some fluorescent materials, once exposed to sunlight or artificial lighting, produce an afterglow in a dark place for a relatively long period of time. Fluorescent materials that exhibit this feature repeatedly are called light-storing fluorescent materials. With increasing awareness of the importance of disaster prevention in recent years, the use of light-storing fluorescent materials, which emit light in a dark place, is expanding throughout the field of disaster prevention. Moreover, the application of light-storing fluorescent materials is broadening due to the use thereof in plastics for the production of plates, sheets, etc.
Heretofore, green-light-emitting ZnS:Cu phosphors have been used as light-storing fluorescent materials, but the properties thereof are unsatisfactory because the afterglow luminosity of such phosphors is not intense enough to be perceived over several tens of hours, and the phosphors are photo degraded by ultraviolet radiation, after which colloidal zinc metal precipitates on the surface of the phosphor crystal, thereby blackening its appearance and significantly impairing its afterglow luminosity. Such degradation is likely to occur under hot and humid conditions. It is difficult to completely prevent this problem. Therefore, the use of ZnS:Cu phosphors in a place involving exposure to direct sunlight, e.g., outdoors, should be avoided.
Japanese Unexamined Patent Publication No. 1995-11250 discloses “a light-storing phosphor in which the mother crystal is of a compound represented by MAl2O4 wherein M is at least one metal element selected from the group consisting of calcium, strontium, and barium”. This light-storing phosphor is described as emitting an afterglow for a considerably longer period of time than the aforementioned sulfur-based phosphors, and having chemical stability and superior long-term photo resistance. In Japanese Unexamined Patent Publication No. 1995-11250, the light-storing phosphor is described as emitting a bluish-purple to green light.
Japanese Unexamined Patent Publication No. 1996-170076 discloses a long-afterglow phosphor represented by the formula MO.a(Al1−bBb)2O3:cR wherein a, b and c are in the ranges given below:    0.5≦a≦10.0,    0.0001≦b≦0.5,    0.0001≦c≦0.2,provided that MO represents at least one divalent metal oxide selected from the group consisting of MgO, CaO, SrO and ZnO; and R represents Eu2+ and at least one additional rare earth element selected from the group consisting of Pr, Nd, Dy and Tm. This invention can be considered as having overcome the notable disadvantage presented by the aforementioned sulfur-based phosphors. In Japanese Unexamined Patent Publication No. 1996-170076, the long-afterglow phosphor is described as emitting a bluish green light.
WO00/11106 discloses “of a rare earth oxysulfide phosphor activated by europium, a red-light-emitting afterglow photoluminescence phosphor represented by the chemical formula: Ln2O2S:Eux,My     0.00001≦x≦0.5    0.00001≦y≦0.3wherein Ln is at least one member selected from the group consisting of Y, La, Gd and Lu; and M is a coactivator and is at least one member selected from the group consisting of Mg, Ti, Nb, Ta and Ga”. This invention can be considered as having overcome the notable disadvantage presented by the aforementioned sulfur-based phosphors. In WO00/11106, the red-light-emitting afterglow photoluminescence phosphor is described as emitting red light.
However, it is very difficult to create an intermediate color of the emission colors of the aforementioned light-storing phosphors, i.e., producing white from green, bluish purple, bluish green and red created by the light-storing phosphors, because the attenuation behavior of the afterglow luminosity and the excitation behavior of each light-storing phosphor are different, and therefore maintaining a white afterglow for a long period of time is almost impossible.
Color control is also difficult for other intermediate colors, due to the difference in the excitation behavior and the attenuation behavior of the afterglow luminosity among the light-storing phosphors. Therefore, there has been no commercial production of phosphors that can create intermediate colors.
Japanese Unexamined Patent Publication No. 1996-151573 discloses “of an aluminate phosphor activated by divalent europium, an afterglow phosphor represented by the chemical formula (Ca1−p−q−r, EupNdqMnr)O.n(Al1−mBm)2O3.kP2O6     wherein 0.0001≦p≦0.5,    0.00005≦q≦0.5,    0.00005≦r≦0.7,    0.0001≦p+q+r≦0.75,    0.0001≦m≦0.5,    0.5≦n≦3.0,    0≦k≦0.21≦r/p≦20”. This aluminate phosphor having a characteristic of producing an afterglow is described as being able to create an afterglow hue that had not been possible with conventional aluminate phosphors having afterglow properties. In Japanese Unexamined Patent Publication No. 1996-151573, the afterglow phosphor is described as emitting white light.
Apparatuses furnished with phosphors include, for example, LEDs and fluorescent lamps. For the purpose of describing common applications of LEDs, the number of devices equipped with liquid crystals is increasing due to the advancement of laptop computers and portable electronic devices. Such liquid crystal devices are furnished with backlights to enable viewing in dark places. Such backlights are required to consume little power while emitting light with high luminosity because low power consumption results in longer operation of liquid crystal devices. Some backlights achieve high luminosity by allowing LED chips, used as a backlight source, to emit light in a planar or like manner. LED chips are used in LED displays. Such LED chips are small and efficiently emit light having vivid colors. LED chips are free from burning out and thus offer a long life. Moreover, LED chips exhibit superior characteristics at the initial stage of their operation and are advantageous in resistance to vibration and repetitive light on/light off operation. With respect to LED chips, further reduction in power consumption is desired for applications such as backlight sources and LED displays. In particular, for guidance lights, the installation of which is mandated by the Fire Service Law enforcement ordinance and the fire prevention ordinances established in municipalities throughout the country, display units are required that can provide sufficient luminosity even when the power supply is weak or when the power supply system has ceased to operate.
Light-emitting devices equipped with an LED and a phosphor excited by the LED are examples of display units that meet such requirements. As taught in, for example, Japanese Unexamined Patent Publication No. 1993-314855, by combining an LED and a light-storing (afterglow-producing) phosphor, display units can maintain their light presentation for a desired period of time due to the afterglow of the phosphor even when their power supply is weak or when their power supply system has ceased to operate. For applications in dark places, such as in a switch panel for an electric light, display units do not have to be lit all the time, and the power consumption can be reduced, by intermittently shutting off the electric light, to take advantage of the light emitted by the LED and the light emitted by a light-storing phosphor that is excited by the LED light.
However, conventional light-emitting devices making use of the combination of an LED chip and a light-storing phosphor produce an afterglow of limited color variation, for example, blue and bluish green. Light-emitting devices that produce an afterglow of many different colors are desired.